A direct current (DC)-DC converter is typically used to step down a power supply voltage to meet the needs of a particular circuit. In many instances, such circuits have variable loads—for example, in DC motors, which are found in innumerable types of electronic products, from medical equipment to automobiles. To conserve power while maintaining the proper voltage supply to the variable load, DC-DC converters often employ pulse width modulation (PWM), in which an input voltage that is rapidly switched on and off is applied to an output filter to regulate the voltage and current supplied to the load in an efficient manner (called “switch-mode power supplies”). The switching action is often performed by a hysteretic comparator, which uses a feedback loop from the power supply output to determine the proper switching duty cycle required to regulate the load.
The output voltage of the power supply, however, typically contains some degree of ripple. The hysteretic comparator's hysteresis trip points may be set to account for this ripple so that the hysteretic comparator changes output state at a desired frequency, resulting in a desired power supply switching frequency. However, the amplitude of the ripple in the output voltage can dynamically increase or decrease, thus causing the hysteretic comparator to change output state too frequently or infrequently. This, in turn, results in an inappropriate power supply switching frequency. Accordingly, a technique for reliably controlling a switch-mode power supply's switching frequency—even in the face of ripple amplitude variations—is desired.